Recently, it has been strongly required that a magnetic recording medium is in conformity with high-density recording. For the requirement, a magnetic recording medium using a magnetic recording thin film as a magnetic layer has been investigated and improvements in a magnetic recording particulate medium which is excellent for such reasons as productivity, durability, corrosion resistance, have been particularly extensively made.
The magnetic recording particulate medium is produced by coating a magnetic coating composition containing a ferromagnetic powder and a binder on a non-magnetic support such as a polyethylene terephthalate film, directly or via other layer, followed by drying to form a magnetic layer.
Hitherto a magnetic recording particulate medium, to improve, in particular, the self-demagnetization loss and the overwrite characteristics in high-frequency recording, it has been conventional to thin a magnetic layer. As the result thereof, a magnetic recording particulate medium having a thickness of the magnetic layer of 0.7 .mu.m or thinner has been proposed. However, in this case, as the result of thinning the magnetic layer, there occurs a problem that the durability and surface property of the magnetic recording medium are deteriorated.
Accordingly, recently, a magnetic recording medium of a double-layer structure that a non-magnetic underlayer containing a non-magnetic powder and a binder is further formed between the magnetic layer and the non-magnetic support has been proposed. The magnetic recording medium of as such a double-layer structure has a feature that after forming the underlayer, the surface form of the underlayer is directly reflected on the surface of the magnetic recording medium. Also, the phenomenon becomes more remarkable in the case of thinning the magnetic layer. Usually, because the non-magnetic underlayer is also formed by coating as the magnetic layer, the coating condition and the drying condition become one factor for determining the surge form of the magnetic recording medium.
About the surface form of the magnetic recording medium, it is known that in particular, at high-density recording, the roughness of the surface of the magnetic recording medium influences the characteristics of the magnetic recording medium. That is, it has been practiced that by smoothing the surface of the magnetic recording medium, the magnetic recording medium is stably brought into contact with a magnetic head to improve the electromagnetic recording characteristics. In this case, as the indicator of the smoothness of the surface of the magnetic recording medium, a surface roughness Ra is used at present.
However, according to our investigations, it has been clarified that by simply smoothing a surface of the magnetic recording medium, that is, by reducing the Ra only, the torque characteristics of rotating components that move the magnetic recording medium in contact with a magnetic head are deteriorated. If the torque characteristics are bad, there occurs a problem that a head adsorption phenomenon occurs. On the other hand, when the surface roughness Ra is relatively increased to improve the torque characteristics, it is in a dilemma that the electromagnetic recording characteristics are reduced.